1. Field of the Invention
The present invention relates to a printed circuit board and, more particularly, to a printed circuit board with low noise.
2. Description of the Related Art
As shown in FIG. 1, a printed circuit board 10 is composed of at least three circuit boards; one layer is a voltage source (power) layer, and another layer is a ground layer, the surface of which (either the top face or the bottom face) is used as a layout for signal lines (as a signal layer). At least one electronic component is placed on the top or the bottom layer of the printed circuit board 10 and its signal pins, voltage source pins, and ground pins are separately connected to other electronic components, the voltage source layer and the ground layer on the surface of the printed circuit board 10. The electronic components can be categorized into analog electronic components and digital electronic components based on their signal characteristics. Analog electronic components include audio chips, amplifiers, power supplies, etc.; digital electronic components include digital signal processors (DSP), microprocessors, image signal (video) drivers, etc. To prevent the noise generated on the voltage source layers from the digital electronic components affecting the voltage source layers for the analog electronic components, an isolation area 11 is provided on the voltage source layer of the printed circuit board 10. The region in the isolation area 11 is called an analog area A1, another region outside of the isolation area 11 is called a digital area A2. The analog area A1 and the digital area A2 have no electrical connection. Therefore, any noise generated on the voltage source layers from the digital electronic components cannot affect the voltage source lines for the analog electronic components, which reduces interference on the printed circuit board. In a similar manner the ground layer of the printed circuit board 10 can use the same structure, and so requires no further description.
However, when the digital electronic components and the analog electronic components need to transmit data to each other, the signal lines are of necessity crossed over the isolation area 11, and the resulting variety of voltage levels (meaning that the voltage levels are discontinuous with respect to the power and ground voltages) will affect the characteristics of signals. Furthermore, different signals can interfere with each other due to electro-magnetic coupling generated by the signal lines crossing the isolation area of the printed circuit board, and the electro-magnetic radiation may exceed a predetermined maximum.
Therefore, it is desirable to provide a printed circuit board with low noise to mitigate and/or obviate the aforementioned problems.